The invention relates to the manufacture of seed tape and also to the closure of polyethylene oxide film upon itself.
Seed tape is made commercially from polyethylene oxide film. This film, though essentially polyethylene oxide, may contain some additional materials (plasticizer, stabilizer, antioxidant, dye, etc.). The film, in order to be operative, is readily water soluble. Advantage of this fact has been taken in sealing the tape after the seeds have been deposited thereon, usually while the tape is formed into a troughed shape. Thus, before the seed is deposited, the tape has been moistened with water, as by spraying water thereon or by applying it from a felted moistener. Then, after the seed has been deposited, the tape is closed on itself under pressure, sealing the tape and enclosing the seed.
An important problem, heretofore unsolved, has been proper control of the amount of moisture imparted to the tape in order to cause its sealing. Excessive moisture has bad effects on both the seed and the tape.
Many seeds are quite sensitive to moisture; an excess of moisture can prematurely trigger the germination process and may result in killing the seed. Seeds which can last for many months if kept in an atmosphere that is sufficiently dry may last only a few weeks if the moisture level is a little higher.
The tape itself is sensitive to the applied moisture, though generally insensitive to atmospheric humidity. The tape is water soluble; so obviously the applied moisture used for sealing must be kept below an amount that would simply dissolve the tape. But that is not all. Tape that becomes much too wet turns milky and tends to stick to itself when wound on a roll, so that it cannot be unwound satisfactorily. Even when not wet to that degree, over-moistened tape tends to stretch while wet--i.e., when the seeds are being put in and the tape sealed-- and then to shrink when it dries, after being wound on a reel. Such shrinking can break some delicate seeds and is generally hard on the strength of the tape. If a stretched wet tape is wound tautly on a card, as is common in certain packaging methods, the card will be bowed in along an arc as the tape dries and shrinks.
Several variables make it difficult to control the amount of moisture used. Each thickness and width of polyethylene oxide tape results in somewhat different requirements as to the amount of moisture needed for sealing. Each size of seed and the spacing between seeds also affects the amount of moisture needed for sealing the film. It should be noted that it is not sufficient simply to seal the film along its edges where it laps over; it is also important that the film between adjacent seeds be sealed, in order to retain the spacing between the seeds and hold them in position.
Moreover, the seed tape is difficult to dry once it is too moist. The tape travels at high speeds, typically several thousand feet per hour, during the manufacturing operation of emplacing the seed and closing the tape. These high speeds make it difficult to dry the tape. High temperatures cannot be used to shorten the drying time, because of the low melting temperature of the plastic, because excessive temperature may damage the seed directly, and also because the creation of water vapor within the sealed tape, which results from heating, causes the tape to balloon and to form a hollow tube, with no sealing between seeds. Consequently, drying must be done at a moderate rate that allows time for the moisture to diffuse out through the tape. Adequate drying in this manner requires a drying chamber in which the tape is dried for at least thirty seconds, and this means several hundred feet of tape travel. Drying chambers as large as this would hamper the production operation, and shorter chambers are inadequate.
At this point, it may be said that there is no easy inexpensive way to measure either the amount of moisture being applied or the amount of moisture remaining in the finished tape. As to the amount applied, when it is sprayed, some evaporates, so that the excess collected does not express by mere subtraction from the amount sprayed how much stayed on the tape. Tests have shown that the completed tape has contained between about 3 and 15 percent moisture, probably averaging around 6 to 7 percent,--a little too high for optimum results.
Experimental data have also shown that when the moisture level in the completed tape is higher than about ten percent, the germination percentage of the seed when planted may be seriously degraded by storage. Thus, if the moisture level is over ten percent for one or two weeks after sealing, gradually drying out, some seeds drop by ten to twenty percent in germination level after six months storage. Seed tapes are of advantage principally when the germination levels are quite high, approaching 100 percent. Therefore, a drop of 10 to 20 percent is undesirable.
Other solvents which can be used to seal polyethylene oxide tape are either toxic, flammable, or both.
Since the tape must be sealed, and moisture has been needed to achieve proper sealing, the problem has been extraordinarily difficult.
The purpose of the present invention is to obtain the needed sealing at present manufacturing speeds while not requiring elaborate controls, excessive drying equipment, or even elaborate inspection methods.